As the performance capabilities of electronics and computer systems increase, the semiconductor devices and supporting packaging substrates which make up these systems grow in complexity. Multilayer ceramic (MLC) substrates used in integrated circuit semiconductor package structures may have over 50 individual layers. Each of these layers may have over 50,000 holes and interconnecting patterns and may in some cases contain up to 100,000 holes. Rapid and accurate machining of these holes is necessary for an economical manufacturing process. Current machining devices include the multiple punch apparatus described in Kranik, U.S. Pat. No. 4,425,829. Other substrate machining methods include a laser drilling system such as that described in U.S. patent application Ser. No. 07/428,686, filed Oct. 30, 1989.
It is essential that each of this multiplicity of holes be accurately and completely machined. If a single hole in any layer of the substrate is improperly machined, it may be necessary to scrap the entire substrate. It is therefore necessary to verify the machining operation at the earliest possible point in the manufacturing sequence.
Methods and apparatus exist in the art for verifying the machining of various substrates and for verifying the patterns on substrates such as printed circuit boards, photomasks, or semiconductor wafers. An early system for verifying punch card holes in a card reader is disclosed in Droege, IBM Technical Disclosure Bulletin Vol. 2, No. 4, December 1959, pp. 98-100. Droege discloses a system in which after reading a card at two successive stations the number of holes read at each station is compared.
Other verifying systems operate by comparing two patterns, which should be identical, to detect the presence of inconsistencies between the patterns. The invention of Sase et al., U.S. Pat. No. 4,680,627 generates images of the patterns on two printed circuit boards and compares those images as a means of verification. Verification systems for photomasks typically analyze two portions of the photomask which are designed to be identical to detect any inconsistencies. Such systems are disclosed in Hara et al., U.S. Pat. No. 4,508,453; Specht et al., U.S. Pat. No. 4,805,123; Wihl, U.S. Pat. No. 4,633,504; Wihl et al., U.S. Pat. No. 4,532,650; and Joseph et al., U.S. Pat. No. 4,448,532.
Bishop, U.S. Pat. No. 4,893,346, discloses an optical inspection system which creates a fictitious image of the object to be verified, determines the allowable variations from this fictitious image, and uses these allowable variations in analyzing subsequent objects. Chadwick et al., U.S. Pat. No. 4,877,326 discloses an inspection apparatus which incorporates a quasi-Lambertian illumination device. U.S. patent application Ser. No. 07/428,686, filed Oct. 30, 1989, discloses a verifier which includes a rapidly scanning laser beam, a photocell, and a comparing circuit.
Strope et al., IBM Technical Disclosure Bulletin Vol 23, No. 9, February 1981, pp. 4076-4077, discloses an optical inspection system which uses a linear array of charge coupled photodiodes. This array produces alternate streams of electrical signals which are then converted to signals which represent either a black or a white level, corresponding to the absence or presence of a hole, respectively.
Broadbent, Jr. et al., U.S. Pat. No. 4,555,798 discloses an optical system for inspecting hole quality which also uses a linear array of equally spaced photodiodes. The photodiodes are on a spacing providing multiple photodiodes for analysis of a single hole as the substrate is stepped in an axis perpendicular to the axis along which the photodiodes are arranged. The photodiodes produce an output corresponding to either a 0 or a 1. The outputs of the photodiodes are then analyzed to generate a hole profile.
Another inspection system utilizing a linear array of photodiodes is disclosed in Sredic, French Patent No. 2,500,925. As in the Broadbent device, Sredic discloses a device in which the photodiode outputs correspond to either a digital 0 or 1. These signals are then transmitted in series and analyzed. In one embodiment the Sredic device can only be used to detect the present or absence of holes. In a second embodiment in which the diode array is denser than the hole spacing the Sredic invention can measure the position and diameter deviations of the holes as well.